1. Field of the Invention
The invention relates to high clarity image bearing sheets that, used with image projectors, provide bright projected images. More specifically, the invention provides transparent image bearing sheets, including coating additives selected to reduce scattering of light by toners and related materials used for the electrophotographic production of colored images. The coated, image-bearing sheets provide projected images having good color saturation, low light scattering, and high contrast due to the clarity and low haze of the sheets.
2. Description of Related Art
Since the introduction of electrophotographic copying and printing machines, using toner powder particles to develop electrostatic images, there has been a continuing emphasis on toner image transfer with faithful, quality fused image reproduction on the surface of a receptor sheet. Initially using black toner powder compositions, transferred to plain paper, electrophotographic imaging technology now extends to the application of colored images to clear films, to produce colored image transparencies suitable for projection using overhead projectors. With each development in technology, a need has arisen to re-visit issues of image quality with recent emphasis on transparency, color saturation, image contrast, edge sharpness, toner fusion and other characteristics that could reduce the acuity and visual impact of a projected image.
Study of the control of image characteristics revealed key requirements for producing optimum images developed by toner powders that were fused with a fuser roller after deposition on a receptor substrate. For example, the quality of the color image depends on the surface flatness including the areas covered by fused toner particles. A poorly fused toner image has multiple surfaces and edges which, upon projection, yield dimming gray tones leading to dull, poor color quality because of incident light scattering at the surfaces and edges. Improved flatness of the image bearing layer may be achieved if a receptor, coated on a film, has sufficient miscibility with a toner powder during image transfer and the toner powder exhibits low melt viscosity during elevated temperature image fusion.
Use of powder toners in electrophotographic copiers and printers is well known in the art. U.S. Pat. No. 2,855,324 discloses thermoplastic coated receptors to which a dry toner image may be transferred by contact under pressure. U.S. Pat. No. 4,071,362 discloses use of a styrene type resin to fuse with thermoplastic toner particles.
U.S. Pat. Nos. 5,208,093, 4,298,309 and 5,635,325 disclose a variety of solutions to achieve miscibility of the coated film with the toner while maintaining low melt viscosity.
U.S. Pat. No. 5,635,325 discloses a core/shell toner for developing electrostatic images including a binder resin, a colorant and an ester wax, wherein the core melts and acts as a release agent during fusing, eliminating the need for silicone based release agents to be applied to the fuser rolls.
U.S. Pat. No. 5,302,439 discloses a recording sheet which comprises a substrate and a coating thereon containing a binder and a material having a melting point of less than about 65.degree. C. and a boiling point of more than about 150.degree. C. and selected from the group consisting alkyl phenones, alkyl ketones, halogenated alkanes, alkyl amines, alkyl anilines, alkyl diamines, alkyl alcohols, alkyl diols, halogenated alkyl alcohols, alkane alkyl esters, saturated fatty acids, unsaturated fatty acids, alkyl aldehydes, alkyl anhydrides, alkanes, and mixtures thereof, and optional traction agent and antistatic agent. Materials from the various groups increase the adhesion of toner powder to the recording sheet.
U.S. Pat. No. 5,451,458 discloses a recording sheet which comprises a substrate and a coating thereon containing a binder selected from polyesters, polyvinyl acetals, vinyl alcohol-vinyl acetal copolymers, polycarbonates, and mixtures thereof, and an additive having a melting point of less than about 65.degree. C. and a boiling point of more than about 150.degree. C. and selected from the group consisting of furan derivatives, cyclic ketones, lactones, cyclic alcohols, cyclic anhydrides, acid esters, phosphine oxides and mixtures thereof, and optional filler, and optional antistatic agent and an optional biocide. The various classes of additives improve image transfer such that almost 100% of the toner powder releases from the imaging drum to the recording sheet.
Previous studies related to the quality of images produced by transfer of toner powder, from imaging drums of electrophotographic copiers and printers to suitable recording sheets, focused attention on the bond formed between the powder and the recording sheet. Having demonstrated sufficient adhesion, measurement of optical density indicated the intensity of the image formed on the recording sheet, as shown by U.S. Pat. No. 5,451,458. Adhesion of toner powder particles and measurement of image density describe image characteristics in relatively crude terms, showing successful toner powder transfer. Although successfully transferred to a transparency sheet, a toner powder image may include defects which, upon projection, become enlarged to cause noticeable image distortion. A need exists for improvement of projected image quality, with emphasis on transparency for optimum light transmission with minimum scattering, high color saturation, image contrast and edge sharpness associated with accurate image transfer and improved toner fusion.